Field of the Invention
The invention lies in the field of communications. The invention relates to a switching matrix for coupling input lines (EL) and output lines (AL) of a switching center to a network node in a communications network, in particular an Asynchronous Transfer Mode ("ATM") communications network with primary switching elements in order to select input signals that arrive through the input lines at the respective primary switching element and to output only the selected input signals on the output side.
Switching matrices at network nodes in a communications network, in particular an ATM communications network, exist. In a conventional switching matrix, 16 input lines, for example, are coupled to 16 output lines, so that input signals arriving at the switching matrix through each of the input lines can be passed or switched to any of the output lines. For example, the switching elements in the switching matrix identify the correct output line to which the input signal is intended to be passed based on information that is defined in a conventional manner, in the cell header of an ATM cell, which represents the input signal or part of the input signal. In particular, it is known for the input signals or a number of input lines, or on a number of transmission channels that are connected on the input side, to be transmitted through a common transmission medium, for example, a glass-fiber cable, optically to the switching matrix. An optical/electrical converter is then located at an appropriate input of the switching matrix and converts the optical signals into electrical signals, which are distributed or switched within the switching matrix. The electrical signals are then passed either individually, or with a number of them jointly, through the input lines to the primary switching elements.
The prior art also includes a switching matrix in which an optical/electrical converter is disposed on a base board. The base board is fitted with a bus structure that connects the optical/electrical converter to the primary switching elements. In the case of the 16/16 switching matrix mentioned above, that is to say a switching matrix having 16 inputs and 16 outputs, through which the signals can arrive at and depart from the switching matrix, respectively, the bus structure distributes the input signals between a total of eight (16/2) switching elements, that is to say primary switching elements, which each have 16 input ports and two output ports. From the output ports of the 16/2 switching elements, the input signals that may be selected by the switching elements are passed as output signals to one electrical/optical converter respectively. At the converter output the signals are passed to a glass-fiber cable having two output channels.
The bus structure of such a switching matrix, which is mounted on a base board, can be produced at a relatively low cost and can be loaded up to an overall digital data rate of about 10 to 15 Gbit/s. In the specific case of the 16/16 switching matrix, for example, it can be loaded up to an overall data rate of 16.times.800 Mbit/s=12.8 Gbit/s.
As communications networks have become increasingly complex, switching matrices have been proposed that can handle a considerably greater overall data rate than 15 Gbit/s. For example, a 64/64 switching matrix has been proposed having four glass-fiber cables on the input side, through each of which input signals from 16 connections or channels can be passed to the switching matrix. Accordingly, the switching matrix has 32 assemblies, each having four primary 16/2 switching elements, with, in each case, one of the four primary 16/2 switching elements in each assembly being allocated to one of four glass-fiber cables on the input side, and being connected to it. The total of eight output ports of the four primary 16/2 switching elements of each assembly are connected to input ports of a secondary switching element of the respective assembly, whose two output ports are in turn connected to one electrical/optical converter respectively in order to output the output signals from the switching matrix.
With such a switching matrix having a high overall data rate, a simple bus structure that, as described above using the example of the 16/16 switching matrix, is mounted on a single base board is no longer sufficient to connect all the inputs of the switching matrix to the primary switching elements. It has, thus, been proposed that optical dividers be provided that, in the signal propagation direction, are disposed on the input side upstream of the optical/electrical converters. At the optical dividers, the input signals on each glass-fiber cable or each glass-fiber cable harness are duplicated, with one of the duplicated input signals respectively being passed to one optical/electrical converter, which is provided at the input of one of a number of base boards each having a bus structure. Each of the base boards or bus structures has a number of optical/electrical converters on the input side, the number being equal to the number of glass-fiber cables on the input side, or to the number of glass-fiber cable harnesses on the input side of the switching matrix. A part of the task of distributing the input signals between the primary switching elements is, thus, taken over by optical dividers and, on the output side, glass-fiber cables connected to them. However, such a configuration has a disadvantage that the optical division results in the light intensity of each input signal being reduced, which means that it is necessary to operate with a relatively high signal light intensity on the transmission paths upstream of the optical dividers, and/or to use high-quality, and thus expensive, optical dividers. Furthermore, a greater number of optical/electrical converters are available than the number of input lines.
It has also been proposed that a considerably more complex bus structure be used than the bus structure disclosed from the 16/16 switching matrix described above, which connects all the inputs of the switching matrix to the required primary switching elements. However, the complex bus structure is mounted on a correspondingly large base board and has a multilayer structure, with conductor tracks of different parts of the bus structure being disposed in each of the layers and having to be insulated from conductor tracks of the other parts or of the other layers, because conductor tracks of the different parts of the bus structure cross over. Furthermore, the capability to use such a complex bus structure in a modular fashion is limited, in particular, due to the relatively high production complexity. Financially, therefore, a complex bus structure cannot be used sensibly for relatively simple switching matrices with relatively low overall data rates.